Standard imaging modalities of small vessels have inherent limitations and cannot characterize cross-sectional wall geometry or transmural vessel wall pathology. Therefore the use of miniaturized intravascular ultrasound has become an emerging imaging technique to characterize artery wall morphology and to monitor transcatheter procedures. We are working with a variety of miniature prototype flexible catheters (1.5mm) capable of generating high radial resolution ultrasound images after placement within the vessel. To study the feasibility and utility of this new imaging system, arteries were imaged in sheep and vessel wall dimensions were correlated with contrast angiography. In addition, ultrasound examination of metallic stents after implantation was performed in an effort to define ultrasound factors which might help in assessing the adequacy of stent implantation. We found a striking correlation between angiographic and ultrasound vessel wall diameter but noted important additional information from ultrasound including cross-sectional area, lumen geometry, transmural thickness and appearance, and pulsatile changes during the cardiac cycle suggesting vessel wall compliance. Metallic stents were easily visualized and several factors relating to stent placement might be of clinical importance. The presence or absence of intraluminal filling defects, the adequacy of circumferential stent expansion, and stent artery wall contact are all variables which can be identified easily using ultrasound techniques. Therefore, in this preliminary in vivo study we feel encouraged that catheter-based ultrasound imaging techniques will be an important modality to help characterized vessel wall geometry and pathology.